Bright zinc



Patented July 27, 1937 UlTE STAT s BRIGHT ZINC Walter E. Meyer, Milford,and Michael F. Dunleavey, Bridgeport, Conn., assignors to GeneralElectric Company, a corporation of New York No Drawing. ApplicationMarch 10, 1936, Serial No. 68,064

Claims.

The present invention relates to a method for producing zinc coatedarticles and more particularly to a bath which may beemployed to producea bright finish on zinc plated articles.

5 Heretofore zinc in the form of an electro-plated coating, althoughoffering a high resistance to oxidation, has been supplanted to a greatextent by other and more expensive metals which offer a more attractiveappearance. We have found however, that it is possible to produce a very.bright zinc coating which in appearance compares favorably with brightcoatings of other and more expensive metals such as cadmium.

In carrying out our invention, we may employ any plated zinc articles.It is preferable however to form the initial zinc coating on afoundation metal in the following manner: The metal to be plated shouldbe cleaned thoroughly and if it is an alloy high in carbon it may befound desirable to give it a sulphuric acid, anodic, electro-platingtreatment. The cleaned article is then electroplated in a bath, theoptimum composition of which is about ozs. per gallon of sodium cyanide,8 to 10 ozs. per gallon of sodium hydroxide and 8 to 10 ozs. per gallonof zinc cyanide. A lower concentration of zinc cyanide results in alower cathodic efliciency and better throwing power, but variations inthe metal concentration do not affect the brightness and platingcharacteristics as markedly as variations in the sodium cyanide andcaustic soda contents. Higher concentrations of sodium cyanide cause adecrease in cathodic efllciency and a slight increase in throwing powerand a noticeable decrease in brightness. Higher and lower concentrationsof caustic soda 'decrease cathodic efliciency and cause a darkening ofthe zinc plate. Any colloidal brightening agent may be added to the bathif desired but we have found sodium thiosulphate to be the mostsatisfactory, the optimum concentration employed being about oz. to thegallon, although amounts up to 2 ozs. per gallon have no detrimentaleffect. Sodium bisulphite up to about 4 cm. per gallon 45 may be addedto the bath if desired and if the cyanide content of the bath is abovethe optimum value the addition of the sodium bisulphite reduces thecyanide content with a resultant increase in brightness of the plate.The optimum 50 current density range for a bright zinc deposit is fromabout 15 to 30 amperes' per square foot. The throwing power increaseswith higher current densities, with lower metal concentrations, or, ingeneral, the factors which cause increased 55 cathodic polarization.

After the zinc coating has been plated on the foundation metal, theplated article is rinsed carefully and then dipped in a brighteningsolution which'consists substantially of 10 to 60 ozs. of chromicacidper gallon, .05 to 5 ozs. of anhydrous sodium sulphate per gallonand .1 to 1 oz. of nitric acid per gallon. After dipping the zinc platedarticle in this solution, it is again rinsed. The optimum values of theingredients of the brightening bath are 40 ozs. of chromic acid pergallon, 2 ozs. per gallon of anhydrous sodium sulphate and .7 oz. pergallon of nitric acid. The resulting zinc coating does not stain and hasa particularly bright and pleasing appearance.

What we claim as new and desire to secure by letters Patent of theUnited States, is:

1. A solution adapted for the production of bright zinc surfaces, saidsolution containing about 10 to 60 ozs. per gallon of chromic acid,about .05 to 5 ozs. per gallon of anhydrous sodium sulphate and about .1to .1 oz. per gallon of nitric acid.

2. A dipping solution for the production of bright zinc coatings, saidsolution consisting substantially of about 40 ozs. per gallon of chromicacid, 2 ozs. per gallon of anhydrous sodium sulphate and .7 oz. pergallon of nitric acid.

3. The method of producing a bright zinc coating which comprisescleaning the article to be plated, immersing the cleaned article in asolution consisting of approximately 10 ozs. per gallon of sodiumcyanide, 8 to 10 ozs. per gallon of sodium hydroxide, 8 to 10 ozs. pergallon of zinc cyanide and about A to 2 ozs. per gallon of sodiumthiosulphate, maintaining the current density of about 15 to 30 amperesin said bath and thereafter immersing the zinc plated article in abrightening solution consisting substantially of 10 to 60 ozs. pergallon of chromic acid, .05 to 5 ozs. per gallon of anhydrous sodiumsulphate and .1 to 1 oz. per gallon of nitric acid;

4. A solution adapted for the production of bright zinc, said solutioncontaining 10 to 60 ozs. per gallon of chromic .acid, .1 to 1 oz. ofnitric acid, and .05 to 5 ozs. per gallon of a compound containing asulphate radical.

5. A solution adapted for the production of bright zinc, said solutioncontaining chromic acid, nitric acid, and a compound containing asulphate radical, the quantity of cromic acid varying from 10 to 60 ozs.per gallon, and the total quantity of said nitric acid and compoundbeing materially less than the quantity of chromic acid present in saidsolution.

WALTER R. MEYER. MICHAEP F. DUNLEAVEY.

